1. Field of the Invention
The present invention relates to an imaging device and a control method for a solid imaging element.
2. Description of Related Art
In the related art, many CMOS image sensors (or CMOS type imaging elements) in solid imaging elements are used to acquire a variety of image data. Generally speaking, photographic devices of MOS type or the like are narrower in latitudes and dynamic ranges than the photographic negative films. In the case of the narrow latitudes and dynamic ranges, the color gradations are decreased such that the dark portions of an image are block pixels whereas the bright portions of the image are white pixels. A logarithmic compression type imaging device exists as a technique for increasing the color gradations.
FIG. 7 shows a circuit diagram of a logarithmic compression type CMOS image sensor. A MOS transistor T4 has its drain and gate connected with each other, so that a photodiode PD is always supplied with a voltage from a voltage line connected with the drain of the MOS transistor T4. At this time, the MOS transistor T4 acts in a sub-threshold region, and the synthetic current of incident photocurrent and dark current, which is photoelectrically converted by the photodiode PD, flows as the drain current of the MOS transistor T4, so that the photodiode PD has its cathode voltage V5 determined by Formula (1) (the detail of which is referred to “Development of Logarithmic Conversion Type CMOS Image Sensor”, KONIKA MINORUTA TECHNOLOGY REPORT vol. 1, 2004, p. 45-50 (Non-Patent Document 1)):V5=Vg−Vt−nKT/q×ln(IdL/WuCox)  (Formula 1).
The cathode voltage of the photodiode PD is connected with the gate of a MOS transistor T5, by which the signal is amplified. The amplified signal is outputted via a row signal line M through a MOS transistor T6 to a post circuit arranged in a matrix state.
The cathode voltage of the photodiode PD for a pixel signal sensitively responds to the drain current of the MOS transistor T4 (i.e., the current to flow through the photodiode PD). Since a temperature T is contained in Equation 1, the dark current component of the drain current changes with the temperature change, so that the ratio between the incident photocurrent and dark current seriously deteriorates.
At a predetermined temperature, the cathode voltage V5 of the photodiode PD changes like a voltage V5a, as shown in FIG. 8. In case the temperature changes, on the other hand, the cathode voltage of the photodiode PD changes like a voltage V5b, as shown in FIG. 8. Even if the quantities of incident lights to a pixel are equal, for an exposure period K2 when the cathode voltage of the photodiode PD changes, the variations of the cathode voltage are different according to the peripheral temperature. As a result, the voltages for a read period K3 when the cathode voltage is substantially stable are different according to the peripheral temperature.
As this method for coping with the problem due to the temperature change, there has been proposed a method for correcting the A/D converted sensor signal outputted from an image sensor, according to the temperature of the image sensor (as referred to WO2002/045414 (Patent Document 1), for example).
If the constitution of Patent Document 1 is adopted, for example, it is necessary to correct the dispersion of the output characteristics in the image sensor, and to correct the offset of output signals. As a result, the constitutions for the individual corrections and the temperature corrections are needed to raise a problem that the circuit constitution is complicates.